Fair Value Measurements (Adjustments Reclassified From Accumulated Balances Other Comprehensive Income (Loss) To Earnings) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments Gain Loss [Line Items]
Costs of contract revenues	$ (154,404)	$ (143,129)
Income tax benefit	(2,274)	(2,653)
Net income (loss)	16,788	$ 4,035
Fuel Hedge Contracts [Member] | Accumulated Net Gain Loss From Designated Or Qualifying Cash Flow Hedges | Reclassification Out Of Accumulated Other Comprehensive Income
Derivative Instruments Gain Loss [Line Items]
Costs of contract revenues	66
Income tax benefit	26
Net income (loss)	$ 40